The increasing use of high frequency electromagnetic radiation in-radar and communication fields has resulted in the need for materials suitable as radiation absorbers, reflectors, filters and polarizers. Of particular interest are materials which can be impedence matched to the transmission medium and used as covers on outer layers of objects to reduce the radar reflectivity of the objects. For example, there is extensive interest in the reduction of the radar cross-section of military hardware such as aircraft, missiles, tanks and ships.
U.S. Pat. No. 4,433,068 to Long et al teaches the use of apparently amorphous polyimide microballoons with filler to improve microwave absorbing properties. Long et al state that the microwave absorption properties of polyimides can be modified and improved by the addition of from about 1 to 50 weight percent microwave absorbing material such as graphite powder, ferrites, metal-ceramic compounds such as ferro titanate or mixtures thereof. U.S. Pat. No. 4,335,180 to Traut discloses the making of a composite high dielectric microwave circuit board using particulate filler (e.g. titania), PTFE and glass fibers. The electronic properties of the board are apparently isotropic.
The disadvantage of these approaches is that a large amount of magnetic or dielectric filler must be added to the composite to raise the magnetic permeability or dielectric constant of the composite as a whole. The addition of large amounts of such fillers may increase the cost of the composites to unacceptable levels. It may also seriously degrade the mechanical, thermal or electrical properties of the composite, rendering it unsuitable for its intended use.
Accordingly, it is an object of the present invention to achieve selected values of magnetic permeability and dielectric constant in a composite while minimizing the use of magnetic and/or dielectric filler materials in the composite.
It is another object of the present invention to achieve selected values of magnetic permeability and dielectric constant in a composite while minimizing the effects on the mechanical, thermal and/or electrical properties of the composite matrix material.
It is another object of the present invention to achieve high magnetic permeability in a composite by the addition of ferrites without adversely affecting the dielectric constant or conductivity of the composite.
It is another object of the present invention to achieve high magnetic permeability in a composite for incident electromagnetic radiation of a particular frequency and polarization.
These and other objects and features will be apparent from the following written description and claims considered with the drawings herein.